Conditionals in C++

The if statement is the simplest form of conditional. It executes a block of code only when the specified condition evaluates to true.

Basic Syntax

#include <iostream>
using namespace std;

int main() {
    int age = 18;
    
    // Simple if statement
    if (age >= 18) {
        cout << "You are an adult." << endl;
    }
    
    // Single-line if (no braces needed for one statement)
    if (age >= 18) cout << "Can vote!" << endl;
    
    // Multiple conditions with logical operators
    int score = 85;
    if (score >= 60 && score <= 100) {
        cout << "Valid passing score!" << endl;
    }
    
    return 0;
}

Condition Evaluation

#include <iostream>
using namespace std;

int main() {
    // In C++, any non-zero value is considered true
    int x = 5;
    
    if (x) {  // 5 is non-zero, so true
        cout << "x is truthy (non-zero)" << endl;
    }
    
    int y = 0;
    if (y) {  // 0 is false, this won't execute
        cout << "This won't print" << endl;
    }
    
    // Pointer check
    int* ptr = nullptr;
    if (ptr) {
        cout << "Pointer is valid" << endl;
    } else {
        cout << "Pointer is null" << endl;
    }
    
    // Common mistake: assignment vs comparison
    int a = 5;
    // if (a = 10) { }  // WRONG! This assigns 10 to a
    if (a == 10) { }    // CORRECT! This compares a to 10
    
    return 0;
}

The if-else statement provides two paths: one for when the condition is true, and another for when it's false.

#include <iostream>
using namespace std;

int main() {
    int number;
    cout << "Enter a number: ";
    cin >> number;
    
    // Basic if-else
    if (number % 2 == 0) {
        cout << number << " is even." << endl;
    } else {
        cout << number << " is odd." << endl;
    }
    
    // Checking positive/negative
    if (number > 0) {
        cout << "Positive number" << endl;
    } else if (number < 0) {
        cout << "Negative number" << endl;
    } else {
        cout << "Zero" << endl;
    }
    
    return 0;
}

Practical Example: Login Validation

#include <iostream>
#include <string>
using namespace std;

int main() {
    string username, password;
    const string correctUser = "admin";
    const string correctPass = "secret123";
    
    cout << "Username: ";
    cin >> username;
    cout << "Password: ";
    cin >> password;
    
    if (username == correctUser && password == correctPass) {
        cout << "Login successful! Welcome, " << username << endl;
    } else {
        cout << "Invalid credentials. Access denied." << endl;
    }
    
    return 0;
}

When you need to check multiple conditions in sequence, use the else if ladder. Only the first matching condition's block will execute.

#include <iostream>
using namespace std;

int main() {
    int score;
    cout << "Enter your score (0-100): ";
    cin >> score;
    
    char grade;
    
    // Grade calculation using else if ladder
    if (score >= 90) {
        grade = 'A';
    } else if (score >= 80) {
        grade = 'B';
    } else if (score >= 70) {
        grade = 'C';
    } else if (score >= 60) {
        grade = 'D';
    } else {
        grade = 'F';
    }
    
    cout << "Your grade: " << grade << endl;
    
    // Additional feedback
    if (grade == 'A' || grade == 'B') {
        cout << "Excellent work!" << endl;
    } else if (grade == 'C' || grade == 'D') {
        cout << "Keep practicing!" << endl;
    } else {
        cout << "Please see your instructor." << endl;
    }
    
    return 0;
}

Order Matters!

#include <iostream>
using namespace std;

int main() {
    int score = 95;
    
    // WRONG order - 95 >= 60 is true, so grade will be D!
    if (score >= 60) {
        cout << "Grade D" << endl;  // This executes first!
    } else if (score >= 70) {
        cout << "Grade C" << endl;
    } else if (score >= 80) {
        cout << "Grade B" << endl;
    } else if (score >= 90) {
        cout << "Grade A" << endl;  // Never reached for 95!
    }
    
    // CORRECT order - check highest thresholds first
    if (score >= 90) {
        cout << "Grade A" << endl;  // This executes for 95
    } else if (score >= 80) {
        cout << "Grade B" << endl;
    } else if (score >= 70) {
        cout << "Grade C" << endl;
    } else if (score >= 60) {
        cout << "Grade D" << endl;
    }
    
    return 0;
}

The switch statement is ideal when you need to compare a variable against multiple constant values. It's often cleaner than long else if chains.

#include <iostream>
using namespace std;

int main() {
    int day;
    cout << "Enter day number (1-7): ";
    cin >> day;
    
    switch (day) {
        case 1:
            cout << "Monday" << endl;
            break;
        case 2:
            cout << "Tuesday" << endl;
            break;
        case 3:
            cout << "Wednesday" << endl;
            break;
        case 4:
            cout << "Thursday" << endl;
            break;
        case 5:
            cout << "Friday" << endl;
            break;
        case 6:
            cout << "Saturday" << endl;
            break;
        case 7:
            cout << "Sunday" << endl;
            break;
        default:
            cout << "Invalid day number!" << endl;
    }
    
    return 0;
}

Fall-Through Behavior

#include <iostream>
using namespace std;

int main() {
    int month;
    cout << "Enter month (1-12): ";
    cin >> month;
    
    int days;
    
    switch (month) {
        // Months with 31 days
        case 1: case 3: case 5: case 7:
        case 8: case 10: case 12:
            days = 31;
            break;
        
        // Months with 30 days
        case 4: case 6: case 9: case 11:
            days = 30;
            break;
        
        // February (simplified)
        case 2:
            days = 28;
            break;
        
        default:
            days = 0;
            cout << "Invalid month!" << endl;
    }
    
    if (days > 0) {
        cout << "Days in month: " << days << endl;
    }
    
    return 0;
}

switch with Characters

#include <iostream>
using namespace std;

int main() {
    char operation;
    double num1, num2, result;
    
    cout << "Enter two numbers: ";
    cin >> num1 >> num2;
    cout << "Enter operation (+, -, *, /): ";
    cin >> operation;
    
    switch (operation) {
        case '+':
            result = num1 + num2;
            cout << num1 << " + " << num2 << " = " << result << endl;
            break;
        case '-':
            result = num1 - num2;
            cout << num1 << " - " << num2 << " = " << result << endl;
            break;
        case '*':
            result = num1 * num2;
            cout << num1 << " * " << num2 << " = " << result << endl;
            break;
        case '/':
            if (num2 != 0) {
                result = num1 / num2;
                cout << num1 << " / " << num2 << " = " << result << endl;
            } else {
                cout << "Error: Division by zero!" << endl;
            }
            break;
        default:
            cout << "Invalid operation!" << endl;
    }
    
    return 0;
}

switch vs if-else

Nested conditionals are if statements inside other if statements. They allow for more complex decision trees but should be used carefully to maintain readability.

#include <iostream>
using namespace std;

int main() {
    int age;
    bool hasLicense;
    
    cout << "Enter your age: ";
    cin >> age;
    cout << "Do you have a license? (1=yes, 0=no): ";
    cin >> hasLicense;
    
    // Nested conditional
    if (age >= 18) {
        if (hasLicense) {
            cout << "You can drive!" << endl;
        } else {
            cout << "You're old enough, but need a license." << endl;
        }
    } else {
        cout << "You're too young to drive." << endl;
        int yearsLeft = 18 - age;
        cout << "Wait " << yearsLeft << " more year(s)." << endl;
    }
    
    return 0;
}

Flattening Nested Conditionals

#include <iostream>
using namespace std;

int main() {
    int age = 20;
    bool hasLicense = true;
    
    // Deep nesting (harder to read)
    if (age >= 18) {
        if (hasLicense) {
            if (age <= 70) {
                cout << "Can drive" << endl;
            }
        }
    }
    
    // Flattened with combined conditions (better)
    if (age >= 18 && hasLicense && age <= 70) {
        cout << "Can drive" << endl;
    }
    
    // Or using early returns in functions
    // (demonstrated in function context)
    
    return 0;
}

Complex Decision Example

#include <iostream>
#include <string>
using namespace std;

int main() {
    string customerType;
    double purchaseAmount;
    int loyaltyYears;
    
    cout << "Customer type (regular/premium): ";
    cin >> customerType;
    cout << "Purchase amount: $";
    cin >> purchaseAmount;
    cout << "Years as customer: ";
    cin >> loyaltyYears;
    
    double discount = 0;
    
    if (customerType == "premium") {
        if (purchaseAmount >= 100) {
            if (loyaltyYears >= 5) {
                discount = 25;  // Premium + $100+ + 5+ years
            } else {
                discount = 20;  // Premium + $100+
            }
        } else {
            discount = 15;  // Premium base
        }
    } else {  // regular customer
        if (purchaseAmount >= 100) {
            if (loyaltyYears >= 3) {
                discount = 15;  // Regular + $100+ + 3+ years
            } else {
                discount = 10;  // Regular + $100+
            }
        } else if (loyaltyYears >= 5) {
            discount = 5;  // Regular loyal customer
        }
    }
    
    cout << "Your discount: " << discount << "%" << endl;
    double finalPrice = purchaseAmount * (1 - discount / 100);
    cout << "Final price: $" << finalPrice << endl;
    
    return 0;
}

The ternary operator ?: is a shorthand for simple if-else statements. It's great for assigning values based on a condition in a single line.

#include <iostream>
#include <string>
using namespace std;

int main() {
    // Syntax: condition ? value_if_true : value_if_false
    
    int age = 20;
    
    // Instead of:
    // string status;
    // if (age >= 18) status = "adult";
    // else status = "minor";
    
    // Use ternary:
    string status = (age >= 18) ? "adult" : "minor";
    cout << "Status: " << status << endl;
    
    // Finding maximum
    int a = 10, b = 20;
    int max = (a > b) ? a : b;
    cout << "Max: " << max << endl;
    
    // Absolute value
    int num = -5;
    int absVal = (num >= 0) ? num : -num;
    cout << "Absolute value: " << absVal << endl;
    
    // In output directly
    int score = 75;
    cout << "Result: " << ((score >= 60) ? "Pass" : "Fail") << endl;
    
    // Nested ternary (use sparingly!)
    int x = 15;
    string size = (x < 10) ? "small" : 
                  (x < 20) ? "medium" : "large";
    cout << "Size: " << size << endl;
    
    return 0;
}

Writing clean, maintainable conditional code is essential. Here are key practices to follow.

1. Always Use Braces

// AVOID: No braces (error-prone)
if (condition)
    doSomething();
    doSomethingElse();  // This ALWAYS executes!

// BETTER: Always use braces
if (condition) {
    doSomething();
    doSomethingElse();
}

2. Use Early Returns

// AVOID: Deep nesting
void processOrder(Order order) {
    if (order.isValid()) {
        if (order.hasStock()) {
            if (order.paymentProcessed()) {
                // Ship order
                ship(order);
            }
        }
    }
}

// BETTER: Early returns (guard clauses)
void processOrder(Order order) {
    if (!order.isValid()) return;
    if (!order.hasStock()) return;
    if (!order.paymentProcessed()) return;
    
    // Ship order
    ship(order);
}

3. Prefer Positive Conditions

// AVOID: Negative conditions
if (!isNotLoggedIn) {
    // confusing!
}

// BETTER: Positive conditions
if (isLoggedIn) {
    // clear and readable
}

4. Avoid Magic Numbers

// AVOID: Magic numbers
if (score >= 90) {
    grade = 'A';
}

// BETTER: Named constants
const int A_THRESHOLD = 90;
const int B_THRESHOLD = 80;

if (score >= A_THRESHOLD) {
    grade = 'A';
} else if (score >= B_THRESHOLD) {
    grade = 'B';
}

5. Use switch for Multiple Values

// AVOID: Long if-else for value matching
if (day == 1) {
    name = "Monday";
} else if (day == 2) {
    name = "Tuesday";
} else if (day == 3) {
    // ... and so on
}

// BETTER: switch statement
switch (day) {
    case 1: name = "Monday"; break;
    case 2: name = "Tuesday"; break;
    case 3: name = "Wednesday"; break;
    // ...
}

6. Keep Conditions Simple

// AVOID: Complex inline condition
if (age >= 18 && age <= 65 && hasLicense && !isSuspended && 
    (carType == "standard" || (carType == "commercial" && hasCommercialLicense))) {
    // hard to read!
}

// BETTER: Extract to meaningful functions or variables
bool isEligibleAge = (age >= 18 && age <= 65);
bool hasValidLicense = hasLicense && !isSuspended;
bool canDriveCarType = (carType == "standard") || 
                       (carType == "commercial" && hasCommercialLicense);

if (isEligibleAge && hasValidLicense && canDriveCarType) {
    // clear intent!
}